How to flip normals with python in blender game engine?
This is only possible via Python. 

Basically you turn the order of the vertices for each single polygon (face) by exchanging the positions of the first and third vertex

flipping triangles:



flipping quads:



You might need to set the vertex normal too to correct the shading.

Attention:

KX_PolyProxy does not provide the right vertices when the mesh consists of a mix of quads and triangles (Blender 2.73 and earlier). I do not know when this gets fixed.